[linux] 01/01: tcp: make challenge acks less predictable (CVE-2016-5696)

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carnil pushed a commit to branch sid
in repository linux.

commit 7184d7bfd94443b6403d71da639ec390224af594
Author: Salvatore Bonaccorso <carnil at debian.org>
Date:   Thu Aug 11 16:46:20 2016 +0200

    tcp: make challenge acks less predictable (CVE-2016-5696)
---
 debian/changelog                                   |  3 +
 .../tcp-make-challenge-acks-less-predictable.patch | 77 ++++++++++++++++++++++
 debian/patches/series                              |  1 +
 3 files changed, 81 insertions(+)

diff --git a/debian/changelog b/debian/changelog
index b087a7b..21f4af0 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -312,6 +312,9 @@ linux (4.6.6-1) UNRELEASED; urgency=medium
   * linux-kbuild: Include headers_install.sh and unifdef (Closes: #832359)
   * Bump ABI to 2
 
+  [ Salvatore Bonaccorso ]
+  * tcp: make challenge acks less predictable (CVE-2016-5696)
+
  -- Ben Hutchings <ben at decadent.org.uk>  Sat, 30 Jul 2016 14:23:58 +0100
 
 linux (4.6.4-1) unstable; urgency=medium
diff --git a/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch b/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch
new file mode 100644
index 0000000..5322d34
--- /dev/null
+++ b/debian/patches/bugfix/all/tcp-make-challenge-acks-less-predictable.patch
@@ -0,0 +1,77 @@
+From: Eric Dumazet <edumazet at google.com>
+Date: Sun, 10 Jul 2016 10:04:02 +0200
+Subject: tcp: make challenge acks less predictable
+Origin: https://git.kernel.org/linus/75ff39ccc1bd5d3c455b6822ab09e533c551f758
+
+Yue Cao claims that current host rate limiting of challenge ACKS
+(RFC 5961) could leak enough information to allow a patient attacker
+to hijack TCP sessions. He will soon provide details in an academic
+paper.
+
+This patch increases the default limit from 100 to 1000, and adds
+some randomization so that the attacker can no longer hijack
+sessions without spending a considerable amount of probes.
+
+Based on initial analysis and patch from Linus.
+
+Note that we also have per socket rate limiting, so it is tempting
+to remove the host limit in the future.
+
+v2: randomize the count of challenge acks per second, not the period.
+
+Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
+Reported-by: Yue Cao <ycao009 at ucr.edu>
+Signed-off-by: Eric Dumazet <edumazet at google.com>
+Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
+Cc: Yuchung Cheng <ycheng at google.com>
+Cc: Neal Cardwell <ncardwell at google.com>
+Acked-by: Neal Cardwell <ncardwell at google.com>
+Acked-by: Yuchung Cheng <ycheng at google.com>
+Signed-off-by: David S. Miller <davem at davemloft.net>
+[carnil: Backported to 4.6: adjust context]
+---
+ net/ipv4/tcp_input.c | 15 ++++++++++-----
+ 1 file changed, 10 insertions(+), 5 deletions(-)
+
+--- a/net/ipv4/tcp_input.c
++++ b/net/ipv4/tcp_input.c
+@@ -87,7 +87,7 @@ int sysctl_tcp_adv_win_scale __read_most
+ EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
+ 
+ /* rfc5961 challenge ack rate limiting */
+-int sysctl_tcp_challenge_ack_limit = 100;
++int sysctl_tcp_challenge_ack_limit = 1000;
+ 
+ int sysctl_tcp_stdurg __read_mostly;
+ int sysctl_tcp_rfc1337 __read_mostly;
+@@ -3460,7 +3460,7 @@ static void tcp_send_challenge_ack(struc
+ 	static u32 challenge_timestamp;
+ 	static unsigned int challenge_count;
+ 	struct tcp_sock *tp = tcp_sk(sk);
+-	u32 now;
++	u32 count, now;
+ 
+ 	/* First check our per-socket dupack rate limit. */
+ 	if (tcp_oow_rate_limited(sock_net(sk), skb,
+@@ -3468,13 +3468,18 @@ static void tcp_send_challenge_ack(struc
+ 				 &tp->last_oow_ack_time))
+ 		return;
+ 
+-	/* Then check the check host-wide RFC 5961 rate limit. */
++	/* Then check host-wide RFC 5961 rate limit. */
+ 	now = jiffies / HZ;
+ 	if (now != challenge_timestamp) {
++		u32 half = (sysctl_tcp_challenge_ack_limit + 1) >> 1;
++
+ 		challenge_timestamp = now;
+-		challenge_count = 0;
++		WRITE_ONCE(challenge_count, half +
++			   prandom_u32_max(sysctl_tcp_challenge_ack_limit));
+ 	}
+-	if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
++	count = READ_ONCE(challenge_count);
++	if (count > 0) {
++		WRITE_ONCE(challenge_count, count - 1);
+ 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPCHALLENGEACK);
+ 		tcp_send_ack(sk);
+ 	}
diff --git a/debian/patches/series b/debian/patches/series
index c3332e1..fd94c31 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -104,6 +104,7 @@ bugfix/x86/kvm-vmx-more-complete-state-update-on-apicv-on-off.patch
 bugfix/all/usb-usbfs-fix-potential-infoleak-in-devio.patch
 bugfix/all/tipc-fix-an-infoleak-in-tipc_nl_compat_link_dump.patch
 bugfix/all/rds-fix-an-infoleak-in-rds_inc_info_copy.patch
+bugfix/all/tcp-make-challenge-acks-less-predictable.patch
 
 # ABI maintenance
 debian/mips-siginfo-fix-abi-change-in-4.6.2.patch

-- 
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